Modul JavaScript: Panduan Pemula

Sekiranya anda seorang pendatang baru dalam JavaScript, jargon seperti "pengikat modul vs. pemuat modul", "Webpack vs Browserify" dan "AMD vs. CommonJS" dapat menjadi cepat.

Sistem modul JavaScript mungkin menakutkan, tetapi memahaminya sangat penting bagi pembangun web.

Dalam catatan ini, saya akan membongkar kata kunci ini untuk anda dalam bahasa Inggeris biasa (dan beberapa contoh kod). Saya harap anda dapat membantu!

Catatan: demi kesederhanaan, ini akan dibahagikan kepada dua bahagian: Bahagian 1 akan menerangkan mengenai modul apa dan mengapa kita menggunakannya. Bahagian 2 (diposting minggu depan) akan membahas apa yang dimaksudkan dengan menggabungkan modul dan pelbagai cara untuk melakukannya.

Bahagian 1: Bolehkah seseorang menerangkan modul apa lagi?

Pengarang yang baik membahagikan buku mereka menjadi bab dan bahagian; pengaturcara yang baik membahagikan program mereka menjadi modul.

Seperti bab buku, modul hanya sekumpulan kata (atau kod, mengikut kesesuaiannya).

Modul yang baik, bagaimanapun, sangat mandiri dengan fungsi yang berbeza, memungkinkan mereka digeser, dikeluarkan, atau ditambahkan seperlunya, tanpa mengganggu sistem secara keseluruhan.

Mengapa menggunakan modul?

Terdapat banyak faedah menggunakan modul yang menyokong pangkalan data yang saling bergantung dan luas. Yang paling penting, pada pendapat saya, adalah:

1) Kebolehlanjutan: Secara definisi, modul adalah serba lengkap. Modul yang dirancang dengan baik bertujuan untuk mengurangkan kebergantungan pada bahagian pangkalan kod sebanyak mungkin, sehingga dapat berkembang dan bertambah baik secara bebas. Mengemas kini satu modul jauh lebih mudah apabila modul dipisahkan dari bahagian kod yang lain.

Kembali ke contoh buku kami, jika anda ingin mengemas kini bab dalam buku anda, akan menjadi mimpi buruk jika perubahan kecil pada satu bab mengharuskan anda untuk mengubah setiap bab yang lain juga. Sebaliknya, anda ingin menulis setiap bab sedemikian rupa sehingga penambahbaikan dapat dilakukan tanpa mempengaruhi bab-bab lain.

2) Namespacing: Dalam JavaScript, pemboleh ubah di luar ruang lingkup fungsi tingkat atas bersifat global (bermaksud, setiap orang dapat mengaksesnya). Oleh kerana itu, biasa berlaku "pencemaran ruang nama", di mana kod yang sama sekali tidak berkaitan berkongsi pemboleh ubah global.

Berkongsi pembolehubah global antara kod yang tidak berkaitan adalah pembangunan yang tidak penting.

Seperti yang akan kita lihat kemudian dalam catatan ini, modul membolehkan kita mengelakkan pencemaran ruang nama dengan mewujudkan ruang peribadi untuk pemboleh ubah kita.

3) Reusability: Mari kita jujur ​​di sini: kita semua telah menyalin kod yang sebelumnya kita tulis ke dalam projek baru pada satu ketika atau yang lain. Sebagai contoh, bayangkan anda menyalin beberapa kaedah utiliti yang anda tulis dari projek sebelumnya ke projek semasa anda.

Itu semua baik dan baik, tetapi jika anda mencari cara yang lebih baik untuk menulis sebahagian kod tersebut, anda harus kembali dan ingat untuk mengemas kini di mana sahaja anda menulisnya.

Ini jelas membuang masa. Bukankah lebih mudah jika ada - tunggu - modul yang boleh kita gunakan berulang kali?

Bagaimana anda boleh memasukkan modul?

Terdapat banyak cara untuk memasukkan modul ke dalam program anda. Mari kita lihat beberapa dari mereka:

Corak modul

Corak Modul digunakan untuk meniru konsep kelas (kerana JavaScript tidak menyokong kelas secara asli) sehingga kita dapat menyimpan kaedah dan pemboleh ubah awam dan swasta di dalam satu objek - serupa dengan bagaimana kelas digunakan dalam bahasa pengaturcaraan lain seperti Java atau Python. Itu membolehkan kita membuat API yang menghadap masyarakat untuk kaedah yang ingin kita dedahkan kepada dunia, sementara masih merangkumi pemboleh ubah dan kaedah peribadi dalam ruang lingkup penutupan.

Terdapat beberapa cara untuk mencapai corak modul. Dalam contoh pertama ini, saya akan menggunakan penutupan tanpa nama. Itu akan membantu kita mencapai matlamat kita dengan meletakkan semua kod kita dalam fungsi tanpa nama. (Ingat: dalam JavaScript, fungsi adalah satu-satunya cara untuk membuat ruang lingkup baru.)

Contoh 1: Penutupan tanpa nama

(function () { // We keep these variables private inside this closure scope var myGrades = [93, 95, 88, 0, 55, 91]; var average = function() { var total = myGrades.reduce(function(accumulator, item) { return accumulator + item}, 0); return 'Your average grade is ' + total / myGrades.length + '.'; } var failing = function(){ var failingGrades = myGrades.filter(function(item) { return item < 70;}); return 'You failed ' + failingGrades.length + ' times.'; } console.log(failing()); }()); // ‘You failed 2 times.’

Dengan konstruk ini, fungsi tanpa nama kita mempunyai persekitaran penilaian sendiri atau "penutupan", dan kemudian kita segera menilai itu. Ini membolehkan kita menyembunyikan pemboleh ubah dari ruang nama induk (global).

Apa yang menarik dari pendekatan ini ialah anda boleh menggunakan pemboleh ubah tempatan di dalam fungsi ini tanpa menimpa pemboleh ubah global yang ada secara tidak sengaja, namun masih mengakses pemboleh ubah global, seperti:

var global = 'Hello, I am a global variable :)'; (function () { // We keep these variables private inside this closure scope var myGrades = [93, 95, 88, 0, 55, 91]; var average = function() { var total = myGrades.reduce(function(accumulator, item) { return accumulator + item}, 0); return 'Your average grade is ' + total / myGrades.length + '.'; } var failing = function(){ var failingGrades = myGrades.filter(function(item) { return item < 70;}); return 'You failed ' + failingGrades.length + ' times.'; } console.log(failing()); console.log(global); }()); // 'You failed 2 times.' // 'Hello, I am a global variable :)'

Perhatikan bahawa kurungan di sekitar fungsi anonim diperlukan, kerana pernyataan yang dimulai dengan fungsi kata kunci selalu dianggap sebagai deklarasi fungsi (ingat, Anda tidak boleh memiliki deklarasi fungsi tanpa nama dalam JavaScript.) Akibatnya, tanda kurung di sekitarnya membuat ekspresi fungsi sebaliknya. Sekiranya anda ingin tahu, anda boleh membaca lebih lanjut di sini.

Contoh 2: Import global

Pendekatan popular lain yang digunakan oleh perpustakaan seperti jQuery adalah import global. Ia serupa dengan penutupan tanpa nama yang baru saja kita lihat, kecuali sekarang kita memasukkan global sebagai parameter:

(function (globalVariable) { // Keep this variables private inside this closure scope var privateFunction = function() { console.log('Shhhh, this is private!'); } // Expose the below methods via the globalVariable interface while // hiding the implementation of the method within the // function() block globalVariable.each = function(collection, iterator) { if (Array.isArray(collection)) { for (var i = 0; i < collection.length; i++) { iterator(collection[i], i, collection); } } else { for (var key in collection) { iterator(collection[key], key, collection); } } }; globalVariable.filter = function(collection, test) { var filtered = []; globalVariable.each(collection, function(item) { if (test(item)) { filtered.push(item); } }); return filtered; }; globalVariable.map = function(collection, iterator) { var mapped = []; globalUtils.each(collection, function(value, key, collection) { mapped.push(iterator(value)); }); return mapped; }; globalVariable.reduce = function(collection, iterator, accumulator) { var startingValueMissing = accumulator === undefined; globalVariable.each(collection, function(item) { if(startingValueMissing) { accumulator = item; startingValueMissing = false; } else { accumulator = iterator(accumulator, item); } }); return accumulator; }; }(globalVariable)); 

Dalam contoh ini, globalVariable adalah satu-satunya pemboleh ubah yang global. Manfaat pendekatan ini daripada penutupan tanpa nama adalah bahawa anda menyatakan pemboleh ubah global dimuka, menjadikannya jelas bagi orang yang membaca kod anda.

Contoh 3: Antara muka objek

Namun pendekatan lain adalah membuat modul menggunakan antara muka objek serba lengkap, seperti:

var myGradesCalculate = (function () { // Keep this variable private inside this closure scope var myGrades = [93, 95, 88, 0, 55, 91]; // Expose these functions via an interface while hiding // the implementation of the module within the function() block return { average: function() { var total = myGrades.reduce(function(accumulator, item) { return accumulator + item; }, 0); return'Your average grade is ' + total / myGrades.length + '.'; }, failing: function() { var failingGrades = myGrades.filter(function(item) { return item < 70; }); return 'You failed ' + failingGrades.length + ' times.'; } } })(); myGradesCalculate.failing(); // 'You failed 2 times.' myGradesCalculate.average(); // 'Your average grade is 70.33333333333333.'

Seperti yang anda lihat, pendekatan ini memungkinkan kita memutuskan pemboleh ubah / kaedah apa yang ingin kita rahsiakan (mis. MyGrades ) dan pemboleh ubah / kaedah apa yang ingin kita dedahkan dengan memasukkannya ke dalam penyata pengembalian (mis. Rata-rata & gagal ).

Example 4: Revealing module pattern

This is very similar to the above approach, except that it ensures all methods and variables are kept private until explicitly exposed:

var myGradesCalculate = (function () { // Keep this variable private inside this closure scope var myGrades = [93, 95, 88, 0, 55, 91]; var average = function() { var total = myGrades.reduce(function(accumulator, item) { return accumulator + item; }, 0); return'Your average grade is ' + total / myGrades.length + '.'; }; var failing = function() { var failingGrades = myGrades.filter(function(item) { return item < 70; }); return 'You failed ' + failingGrades.length + ' times.'; }; // Explicitly reveal public pointers to the private functions // that we want to reveal publicly return { average: average, failing: failing } })(); myGradesCalculate.failing(); // 'You failed 2 times.' myGradesCalculate.average(); // 'Your average grade is 70.33333333333333.'

That may seem like a lot to take in, but it’s just the tip of the iceberg when it comes to module patterns. Here are a few of the resources I found useful in my own explorations:

  • Learning JavaScript Design Patterns by Addy Osmani: a treasure trove of details in an impressively succinct read
  • Adequately Good by Ben Cherry: a useful overview with examples of advanced usage of the module pattern
  • Blog of Carl Danley: module pattern overview and resources for other JavaScript patterns.

CommonJS and AMD

The approaches above all have one thing in common: the use of a single global variable to wrap its code in a function, thereby creating a private namespace for itself using a closure scope.

While each approach is effective in its own way, they have their downsides.

For one, as a developer, you need to know the right dependency order to load your files in. For instance, let’s say you’re using Backbone in your project, so you include the script tag for Backbone’s source code in your file.

However, since Backbone has a hard dependency on Underscore.js, the script tag for the Backbone file can’t be placed before the Underscore.js file.

As a developer, managing dependencies and getting these things right can sometimes be a headache.

Another downside is that they can still lead to namespace collisions. For example, what if two of your modules have the same name? Or what if you have two versions of a module, and you need both?

So you’re probably wondering: can we design a way to ask for a module’s interface without going through the global scope?

Fortunately, the answer is yes.

There are two popular and well-implemented approaches: CommonJS and AMD.

CommonJS

CommonJS is a volunteer working group that designs and implements JavaScript APIs for declaring modules.

A CommonJS module is essentially a reusable piece of JavaScript which exports specific objects, making them available for other modules to require in their programs. If you’ve programmed in Node.js, you’ll be very familiar with this format.

With CommonJS, each JavaScript file stores modules in its own unique module context (just like wrapping it in a closure). In this scope, we use the module.exports object to expose modules, and require to import them.

When you’re defining a CommonJS module, it might look something like this:

function myModule() { this.hello = function() { return 'hello!'; } this.goodbye = function() { return 'goodbye!'; } } module.exports = myModule;

We use the special object module and place a reference of our function into module.exports. This lets the CommonJS module system know what we want to expose so that other files can consume it.

Then when someone wants to use myModule, they can require it in their file, like so:

var myModule = require('myModule'); var myModuleInstance = new myModule(); myModuleInstance.hello(); // 'hello!' myModuleInstance.goodbye(); // 'goodbye!'

There are two obvious benefits to this approach over the module patterns we discussed before:

1. Avoiding global namespace pollution

2. Making our dependencies explicit

Moreover, the syntax is very compact, which I personally love.

Another thing to note is that CommonJS takes a server-first approach and synchronously loads modules. This matters because if we have three other modules we need to require, it’ll load them one by one.

Now, that works great on the server but, unfortunately, makes it harder to use when writing JavaScript for the browser. Suffice it to say that reading a module from the web takes a lot longer than reading from disk. For as long as the script to load a module is running, it blocks the browser from running anything else until it finishes loading. It behaves this way because the JavaScript thread stops until the code has been loaded. (I’ll cover how we can work around this issue in Part 2 when we discuss module bundling. For now, that’s all we need to know).

AMD

CommonJS is all well and good, but what if we want to load modules asynchronously? The answer is called Asynchronous Module Definition, or AMD for short.

Loading modules using AMD looks something like this:

define(['myModule', 'myOtherModule'], function(myModule, myOtherModule) { console.log(myModule.hello()); });

What’s happening here is that the define function takes as its first argument an array of each of the module’s dependencies. These dependencies are loaded in the background (in a non-blocking manner), and once loaded define calls the callback function it was given.

Next, the callback function takes, as arguments, the dependencies that were loaded — in our case, myModule and myOtherModule — allowing the function to use these dependencies. Finally, the dependencies themselves must also be defined using the define keyword.

For example, myModule might look like this:

define([], function() { return { hello: function() { console.log('hello'); }, goodbye: function() { console.log('goodbye'); } }; });

So again, unlike CommonJS, AMD takes a browser-first approach alongside asynchronous behavior to get the job done. (Note, there are a lot of people who strongly believe that dynamically loading files piecemeal as you start to run code isn’t favorable, which we’ll explore more when in the next section on module-building).

Aside from asynchronicity, another benefit of AMD is that your modules can be objects, functions, constructors, strings, JSON and many other types, while CommonJS only supports objects as modules.

That being said, AMD isn’t compatible with io, filesystem, and other server-oriented features available via CommonJS, and the function wrapping syntax is a bit more verbose compared to a simple require statement.

UMD

For projects that require you to support both AMD and CommonJS features, there’s yet another format: Universal Module Definition (UMD).

UMD essentially creates a way to use either of the two, while also supporting the global variable definition. As a result, UMD modules are capable of working on both client and server.

Here’s a quick taste of how UMD goes about its business:

(function (root, factory) { if (typeof define === 'function' && define.amd) { // AMD define(['myModule', 'myOtherModule'], factory); } else if (typeof exports === 'object') { // CommonJS module.exports = factory(require('myModule'), require('myOtherModule')); } else { // Browser globals (Note: root is window) root.returnExports = factory(root.myModule, root.myOtherModule); } }(this, function (myModule, myOtherModule) { // Methods function notHelloOrGoodbye(){}; // A private method function hello(){}; // A public method because it's returned (see below) function goodbye(){}; // A public method because it's returned (see below) // Exposed public methods return { hello: hello, goodbye: goodbye } }));

For more examples of UMD formats, check out this enlightening repo on GitHub.

Native JS

Phew! Are you still around? I haven’t lost you in the woods here? Good! Because we have *one more* type of module to define before we’re done.

As you probably noticed, none of the modules above were native to JavaScript. Instead, we’ve created ways to emulate a modules system by using either the module pattern, CommonJS or AMD.

Fortunately, the smart folks at TC39 (the standards body that defines the syntax and semantics of ECMAScript) have introduced built-in modules with ECMAScript 6 (ES6).

ES6 offers up a variety of possibilities for importing and exporting modules which others have done a great job explaining — here are a few of those resources:

  • jsmodules.io
  • exploringjs.com

What’s great about ES6 modules relative to CommonJS or AMD is how it manages to offer the best of both worlds: compact and declarative syntax and asynchronous loading, plus added benefits like better support for cyclic dependencies.

Probably my favorite feature of ES6 modules is that imports are live read-only views of the exports. (Compare this to CommonJS, where imports are copies of exports and consequently not alive).

Here’s an example of how that works:

// lib/counter.js var counter = 1; function increment() { counter++; } function decrement() { counter--; } module.exports = { counter: counter, increment: increment, decrement: decrement }; // src/main.js var counter = require('../../lib/counter'); counter.increment(); console.log(counter.counter); // 1

In this example, we basically make two copies of the module: one when we export it, and one when we require it.

Moreover, the copy in main.js is now disconnected from the original module. That’s why even when we increment our counter it still returns 1 — because the counter variable that we imported is a disconnected copy of the counter variable from the module.

So, incrementing the counter will increment it in the module, but won’t increment your copied version. The only way to modify the copied version of the counter variable is to do so manually:

counter.counter++; console.log(counter.counter); // 2

On the other hand, ES6 creates a live read-only view of the modules we import:

// lib/counter.js export let counter = 1; export function increment() { counter++; } export function decrement() { counter--; } // src/main.js import * as counter from '../../counter'; console.log(counter.counter); // 1 counter.increment(); console.log(counter.counter); // 2

Cool stuff, huh? What I find really compelling about live read-only views is how they allow you to split your modules into smaller pieces without losing functionality.

Then you can turn around and merge them again, no problem. It just “works.”

Looking forward: bundling modules

Wow! Where does the time go? That was a wild ride, but I sincerely hope it gave you a better understanding of modules in JavaScript.

In the next section I’ll walk through module bundling, covering core topics including:

  • Why we bundle modules
  • Different approaches to bundling
  • ECMAScript’s module loader API
  • …and more. :)

NOTE: To keep things simple, I skipped over some of the nitty-gritty details (think: cyclic dependencies) in this post. If I left out anything important and/or fascinating, please let me know in the comments!